1. Field of the Invention
The present invention relates in general to protective relays, and more specifically to protective relays for protecting electrical machines connected to an electrical power system from the harmful affects of sub-synchronous oscillations.
2. Description of the Prior Art
The existence of asynchronous oscillations, i.e., oscillations at frequencies other than the generator's normal frequency, in power systems and series-compensated transmission lines has been known for years. Asynchronous oscillations occur as a result of the resonnace of the system's capacitive and inductive components in response to transient conditions such as switching surges and fault initiation and clearing. Usually the oscillations are quickly damped by resistive losses, and do not precipitate reactions at resonant frequencies elsewhere in the system.
As the series capacitor compensation technique is increasingly employed for optimizing transmission capability of long high-voltage and extra-high-voltage lines, the expectancy that the natural or resonant frequencies of the system could and would fall below the 60 Hz synchronous frequency of the system is greatly increased. When this happens, mechanical resonance response in the generating equipment, via the stator windings, may be triggered.
Once an electrically-initiated mechanical oscillation at a resonant frequency (i.e., a modal frequency) occurs, sustained or growing sub-synchronous oscillations can be expected in a fashion similar to that of a positive feedback system. This effect can lead to eventual malfunction or destruction of the generating equipment. Turbine-generators have been severely damaged by shaft torques produced by electrically initiated mechanical system. Damaging oscillations may build slowly due to interaction between the electrical power system and the mechanical system of the turbine-generator, or a transient of high magnitude, such as due to a short circuit on series compensated transmission line, may provide a "shock" torque which initiates a sub-synchronous oscillation of the mechanical system at a resonant frequency.
Many different approaches have been used to attempt solution of the problem presented by sub-synchronous oscillations. For example, insertion of electrical power filters into the electrical line to block or dampen sub-synchronous currents has been utilized. Also, additional control has been provided for series capacitors to reduce short circuit current magnitudes. Additional control, by increasing system damping, has also been provided in the excitation system of the turbine generator. These approaches to solving the problem, however, may malfunction or fail. Such failure may, for example, be the result of filter detuning, drift of the electrical power system from the synchronous frequency, or a failure of an initiating signal to reach the series capacitor control. Obviously, since the problem is very complex, the solution may not protect the system against sub-synchronous resonance from all possible causes. Thus, it would be desirable to back-up any solution to the sub-synchronous resonance problem with a protective relay to detect an unacceptable sub-synchronous oscillation condition and either provide a trip signal for a circuit breaker to remove the apparatus to be protected from the line, or initiate some other form of corrective action.
A protective relay for such a function must distinguish between sub-synchronous oscillations which are potentially harmful and those which are not. A turbine-generator should be tripped from the line only when damage-producing oscillation thereof is imminent. Since such oscillations may be produced in a very short period of time, the processing speed of the relay must be sufficiently fast to provide the detection and corrective action before actual damage begins. The protective relay must also have a wide operating range and a sensitivity to detect 0.01 p.u. (per unit) of sub-synchronous current in the presence of a synchronous 60 Hz current of many p.u. The sub-synchronous current may also have a magnitude of many p.u.
U.S. Pat. No. 4,125,884, which patent is assigned to the same assignee of the present invention, discloses a monitoring apparatus for providing a signal responsive to the sub-synchronous current flowing in an electrical power system. The signal responsive to a current in one of the phases of a three-phase electrical power system is multiplied by a reference signal having a synchronous frequency and a constant amplitude. The product contains components having frequencies equal to: (1) twice the synchronous frequency, (2) the sum of the synchronous frequency and each sub-synchronous frequency, and (3) the difference between the synchronous frequency and each sub-synchronous frequency. A wide-band or bandpass filter passes component (3) which relates directly to sub-synchronous oscillation frequencies, while substantially blocking or attenuating components (1) and (2).
U.S. Pat. No. 4,106,071, which patent is also assigned to the same assignee as the present invention, discloses a detection apparatus for processing a signal responsive to the sub-synchronous current in an electrical power system, such as the signal (3) discussed hereinabove. This detection apparatus provides a trip signal in response to a growing oscillation in the frequency range of interest when the magnitude of the oscillation exceeds a predetermined threshold.
U.S. Pat. No. 4,208,687, which is also assigned to the same assignee as the present invention, discloses a three-phase approach to the monitoring and detection functions of a sub-synchronous current protective relay. Sequence filters develop signals related to the positive sequence component of any sub-synchronous oscillation in a three-phase electrical power system, and narrow-band filters examine the signals for oscillations at preselected frequencies known to be critical mechanical resonant frequencies of the apparatus to be protected. This patent recognizes that instead of examining only sub-synchronous oscillations growing in magnitude, that an electrical transient of a particular frequency which exceeds a predetermined magnitude may shock the mechanical system of a turbine-generator into mechanical resonance. Thus, corrective action can be taken earlier than with a protective relay which examines only growing oscillations.
U.S. Pat. No. 4,218,718, which is also assigned to the same assignee as the present invention, provides means for reducing the cost of the apparatus of U.S. Pat. No. 4,208,687 and provides certain functional advantages over this apparatus. The invention of U.S. Pat. No. 4,218,718, provides signals representative of the positive sequence component of the sub-synchronous current by multiplying each of three signals related to the three line-currents by different reference signals, each reference signal having a constant amplitude and synchronous frequency. Linear summing and wide-band filtering of the resulting products produces a signal representative of the positive sequence component of any sub-synchronous currents flowing in the associated three-phase electrical power system. The output signal of the wide-band amplifier is processed in a two-stage detection circuit responsive to the electrical transients from the line. The first stage provides an output signal when the output of the wide-band filter exceeds a first predetermined magnitude of relatively large value. The second stage operates when the first predetermined magnitude is not exceeded, but a predetermined second magnitude, smaller than the first, is exceeded. Exceeding the second magnitude initiates a pattern signal for comparison with the output of the wide-band filter. If the output of the wide-band filter exceeds the pattern signal, the second stage provides an output signal. The output signal of the wide-band filter is also processed by detection circuitry for detecting growing oscillations, and may include a narrow-band filter and detection circuitry for each frequency of interest.